The goal of this project is to obtain a better understanding of the role age plays in the acquisition of the grammatical structure of a second language. There is good descriptive evidence from research in the last 15 years that ultimate attainment in the second language is strongly negatively correlated with age of acquisition, but the reasons for this correlation are poorly understood. Yet, a better understanding of age- related difficulties in second language learning is of great importance for the improvement of immigrants' language learning, which has a strong impact on their educational and overall well being. The specific contribution of this study is two-fold. First, it seeks to establish the different nature of language learning mechanisms is a function of age by a quantitative analysis of the interaction between age and language learning mechanisms as a function of age by a quantitative analysis of the interaction between age and language learning aptitude. Secondly, it seeks to provide a deeper qualitative explanation of the nature of this interaction by establishing which grammar structures are affected and how. In order to avoid the confounds between several linguistic variables that are unavoidable in a given language, the acquisition of two radically different languages (English in the U.S. and Hebrew in Israel) by native speakers of the same first language, Russian, will be analyzed in parallel. The level of attainment in each second language will be assessed by means of a grammatically judgement task, designed to test a variety of morphosyntactic structures.